The invention relates to a monomode optical transmission fiber having an end portion with a tapered core and cladding. The invention also relates to a method of manufacturing such a fiber.
Such a fiber and method are described in an article by H. Kuwahara et al entitled "Efficient coupling from semiconductor lasers into single-mode fibers with tapered hemispherical ends", (Applied Optics, Vol. 19, No. 15, August 1980, pages 2578-2583). The tapered end portion of the transmission fiber described in this article has a rounded tip with a diameter of approximately 25 microns. This tip acts as a lens. As described in the article, transmission fibers having tapered cores and claddings, have substantially better coupling efficiency, higher permissible misalignment of the fiber relative to the radiation source (a diode laser), and reduced feedback of radiation to the radiation source (as a result of reflections inside the fiber) as compared with transmission fibers having straight and flat end portions.
The coupling efficiency is to be understood to mean the quotient of the radiation intensity received from the source by the transmission fiber divided by the total radiation intensity emitted by this source.
As a result of refection, a part of the radiation emitted by the diode laser may return into the laser. This feedback radiation may give rise to undesirable modulation of the radiation intensity emitted by the diode laser.
The coupling efficiency attainable by a tapered monomode transmission fiber, which is approximately 25 to 30%, is higher than that of a transmission fiber with a straight and flat end portion. However, this coupling efficiency is still inadequate for practical purposes. The cause of this low coupling efficiency is the fact that the refractive index of the fiber end portion is comparatively low, namely equal to that of the fiber core, consequently, the lens formed by this end portion has a comparatively low numerical aperture and exhibits comparatively large spherical aberration.